Decorative laminates have been used as a surfacing material for many years, in both commercial and residential applications, where pleasing aesthetic effects in conjunction with desired functional behavior (such as superior wear, heat and stain resistance, cleanability and cost) are preferred. Typical applications have historically included, while not limited to, furniture, kitchen countertops, table tops, store fixtures, bathroom vanity tops, cabinets, wall paneling, office partitions, and the like.
More recently, the applications for decorative laminates have been expanded to include their use as a flooring material in lieu of more expensive real wood, stone or ceramic tile, less sanitary and rugged carpeting, as well as less aesthetically attractive vinyl tile or linoleum-like products. However, as discussed in more detail below, existing decorative laminates are not particularly suited in applications where there is repeated or prolonged exposure to moisture and/or water due to their intrinsic hydrophilic properties. Such existing laminates have therefore been primarily limited to residential applications having dry conditions. Accordingly, as discussed further below, there is a need for a decorative laminate that can be used where there is repeated or prolonged exposure to moisture and/or water, thereby overcoming the deficiencies present in existing decorative laminates.
In general, decorative laminates can be classified into two broad categories, namely high pressure decorative laminates (HPDL) and low pressure decorative laminates (LPDL). As defined by the industry's governing body, the National Electrical Manufacturers Association (NEMA) in their Standards Publication LD 3-1995, high pressure decorative laminates are manufactured or “laminated” under heat and a specific pressure of more than 750 psig. Conversely, low pressure decorative laminates are typically manufactured at about 300 to 600 psig specific pressure to avoid excessive crushing of their substrate material. The other broad distinction between high pressure and low pressure decorative laminates is that the former are generally relatively thin, typically comprising a decorative surface and a phenolic resin impregnated kraft paper core, and are not self supporting as manufactured. As such they are normally bonded, with a suitable adhesive or glue, to a rigid substrate such as a particleboard or medium density fiberboard (MDF), as a separate step during final fabrication of the end product. Conversely, low pressure decorative laminates are typically comprised of a similar type of decorative surface, without the supporting core layer, which is bonded to a substrate such as particleboard or MDF in a single laminating or “pressing” operation during its manufacture.
Both high pressure and low pressure decorative laminates have historically been manufactured in heated, flat-bed hydraulic presses. With the exception of some newer types of processing equipment, high pressure laminates are typically pressed as multiple sheets in press “packs” or “books” in a multi-opening press (which is usually steam or high pressure hot water heated, and water cooled), with a 30 to 60 minute thermal cycle and 130° C. to 150° C. top temperature. On the other hand, low pressure decorative laminates are typically pressed as a single sheet or “board” in a single opening press (which is usually thermoil or electrically heated) using an isothermal, hot discharge “short cycle” of 20 to 60 seconds with press heating platen temperatures of 170° C. to 220° C. Continuous laminating or “double belt” presses for decorative laminate manufacture blur the above distinctions somewhat, in that their “cycle” times and temperatures are similar to those employed for low pressure decorative laminates. In such a process, pressures are intermediate, typically in the range of 300 to 800 psig, while the continuous laminates themselves are relatively thin, without direct bonding to a substrate material and thus requiring a second fabrication step to do so as is the case with conventional high pressure decorative laminates. The process and product dissimilarities delineated above, as well as more subtle process differences, will be appreciated by those versed in the art.
High pressure decorative laminates are generally comprised of a decorative sheet layer, which is either a solid color or a printed pattern, over which is optionally placed a translucent overlay sheet, typically employed in conjunction with a print sheet to protect the print's ink line and enhance abrasion resistance, although an overlay can be used to improve the abrasion resistance of a solid color as well. A solid color sheet typically consists of alpha cellulose paper containing various pigments, fillers and opacifiers, generally with a basis weight of 50 to 120 pounds per 3000 square foot ream. Similarly, print base papers are also pigmented and otherwise filled alpha cellulose sheets, usually lightly calendered and denser than solid color papers to improve printability, and lower in basis weight at about 40 to 75 pounds per ream, onto which surface is rotogravure or otherwise printed a design using one or more inks. Conversely, overlay papers are typically composed of highly pure alpha cellulose fibers without any pigments or fillers, although they can optionally be slightly dyed or “tinted”, and are normally lighter in basis weight than the opaque decorative papers, in the range of 10 to 40 pounds per ream.
For high wear applications (such as flooring), it is often desirable to have a more highly wear resistant top layer. Accordingly, the overlay papers may contain hard, abrasive, mineral particles such as silicon dioxide (silica), and preferably aluminum oxide (alumina), which is included in the paper's furnish during the papermaking process. Alternatively, the abrasive particles can be coated on the surface of the overlay or decorative papers, during the “treating” process described below, prior to the final lamination step. Further, the abrasive particles can be added to the resin which is used to impregnate the overlay or decorative layers, thus causing the abrasive particles to be deposited on, and to a lesser extent, dispersed within such layers. As is known in the art, if the abrasive particles are deposited on the decorative layer, a separate overlay layer may not be necessary.
Typically, these overlay and decorative print and solid color surface papers are treated, or impregnated, with a melamine-formaldehyde thermosetting resin, which is a condensation polymerization reaction product of melamine and formaldehyde, to which can be co-reacted or added a variety of modifiers, including plasticizers, flow promoters, catalysts, surfactants, release agents, or other materials to improve certain desirable properties during processing and after final press curing, as will be understood by those skilled in the art. As with melamine-formaldehyde resin preparation and additives thereto, those versed in the art will also appreciate that other polyfunctional amino and aldehydic compounds can be used to prepare the base resin, and other thermosetting polymers, such as polyesters or acrylics, may be useful as the surface resin for certain applications. It is common practice, particularly in low pressure processes, to treat the decorative paper, and optionally a high wear abrasive loaded overlay, with a coreacted melamine-urea-formaldehyde (MUF) resin, or a blend of a melamine-formaldehyde (MF) resin and urea-formaldehyde (UF) resin, where the urea serves as an inexpensive, low cost resin solids extender. However, in the practice of the present invention, which is directed primarily to moisture resistant flooring applications, inclusion of urea, in any form, in the surface resin should be avoided if the best moisture and water resistance of the decorative laminate assembly is to be achieved. It will be appreciated, however, that urea can be used in the practice of the present invention.
Optionally, an untreated decorative paper can be used in conjunction with a treated overlay, provided the overlay contains sufficient resin to flow into and contribute to the adjacent decorative layer during the laminating process heat and pressure consolidation so as to effect sufficient interlaminar bonding of the two, as well as bonding of the decorative layer to the core. The equipment used to treat these various surface papers is commercially available and well known to those skilled in the art. The papers are normally treated to controlled, predetermined resin contents and volatile contents for optimum performance as will be well understood by those versed in the art, with typical resin contents in the ranges of 64–80%, 45–55% and 35–45% for overlay, solid color and print (unless used untreated) papers respectively, and all with volatile contents of about 5–10%. Overlay and decorative surface papers used with a low pressure process usually employ higher resin contents and catalyst concentrations (and/or stronger catalysts) to compensate for the lower pressure and resultant poorer resin flow, and the short thermal cure cycle, during the pressing operation.
The surface papers (ie., the overlay and decorative layers) of a high pressure decorative laminate are simultaneously bonded to the core during the pressing operation. The core of a conventional high pressure decorative laminate is typically comprised of a plurality of saturating grade kraft paper “filler” sheets, which have been treated or impregnated with a phenol-formaldehyde resin, which also simultaneously fuse and bond together during the laminating process, forming a consolidated, multi-lamina unified composite or laminate. Phenol-formaldehyde resins are condensation polymerization reaction products of phenol and formaldehyde. Again, those versed in the art will appreciate that a variety of modifiers such as plasticizers, extenders and flow promoters can be co-reacted with, or added to, the phenol-formaldehyde resin, that other phenolic and aldehydic compounds can be used to prepare the base resin, or that other types of thermosetting resins such as epoxies or polyesters may be used. A phenol-formaldehyde resin, however, is generally preferred in the manufacture of conventional high pressure decorative laminates, as is the use of a saturating grade kraft paper, generally with a basis weight of 70–150 pounds per ream, although other materials such as linerboard kraft paper, natural fabrics, or woven or nonwoven glass, carbon or polymeric fiber clothes or mats may also be used as the core layer, either by themselves or in combination with kraft paper. In any case, these core layers must either be treated with a resin that is chemically compatible with the “primary” filler resin (and surface resin if used adjacent to it), or if used untreated, sufficient resin must be made available from adjacent filler plies to contribute to it and insure adequate interlaminar bonding. The filler resin preparation procedures, and filler treating equipment and methodologies, are also well known to those skilled in the art. With a conventional low pressure process, typically a core layer is not used, and the decorative surface components are bonded directly to a substrate material rather than to an intermediate core layer.
During the HPDL laminating or pressing operation, the various surface and filler sheets or laminae are cured under heat and pressure, fusing and bonding them together into a consolidated, unitary laminate mass, albeit asymmetric in composition throughout its thickness. As mentioned previously, typically this process is accomplished in a multi-opening, flat bed hydraulic press between essentially inflexible, channeled platens capable of being heated and subsequently cooled while under an applied pressure.
Typically in such a press, back-to-back pairs of collated laminate assemblies (with means of separation as described below), each consisting of a plurality of filler sheets and one or more surface sheets, are stacked in superimposed relationship between rigid press plates or “cauls”, with the surfaces adjacent to the press plates. As is known in the art, such press plates are typically fashioned from a heat-treatable, martensitic stainless steel alloy such as AISI 410, and can have a variety of surface finishes which they impart directly to the laminate surface during the pressing operation, or they can be used in conjunction with a non-adhering texturing/release sheet positioned between the laminate surface components and the press plate, which will impart a selected finish to the laminate surface during pressing as well (and is later stripped off and discarded).
While martensitic stainless steel press plates are most commonly used in the manufacture of high pressure decorative laminate, optionally chrome plated to enhance their wear resistance and releasibility, austenitic stainless steels such as AISI 304, or other metal alloys such as brass, either with optional chrome plating, can also be employed, as can heat treatable wrought aluminum alloys, for example 6061 T6 temper, which surface may be anodized to increase its hardness and wear resistance. In addition, nonmetallic press plates or cauls may also be used advantageously. Such plates can be comprised of fully cured materials such as phenolic resin treated kraft paper, epoxy resin treated woven glass cloth, epoxy resin treated carbon fiber mat, or the like compositions. These plates can be optionally clad with a stainless steel or aluminum foil, which further optionally can be respectively chrome plated or anodized for improved wear resistance. Metallic press plates are typically manufactured by buffing and polishing, chemical etching, mechanical embossing, machining, shot peening, or combinations thereof, depending on the texture and surface finish desired, while the composite press plates are typically produced by a heat and pressure consolidation, i.e. lamination, and embossing process such as that described in U.S. Pat. No. 3,718,496 Willard. Release/texturing papers can be, or may have to be, used in conjunction with a particular type of press plate depending on its intrinsic self-release characteristics as well as the final laminate finish desired.
Typically, several pairs of laminate assemblies or “doublets” are interleaved between several press plates, supported by a carrier tray, to form a press pack or “book”. The laminate pairs between the press plates are usually separated from each other by means of a non-adhering material such as a wax or silicone coated paper, or biaxially oriented polypropylene (BOPP) film, which are commercially available. Alternatively, the backmost face of one or both of the laminates' opposed filler sheets in contact with each other is coated with a release material such as a wax or fatty acid salt. Each press pack, so constructed, is then inserted, by means of its carrier tray, into an opening or “daylight” between two of the heating/cooling platens of the multi-opening, high pressure flat bed press. The press platens are typically heated by direct steam, or by high pressure hot water, the latter usually in a closed-loop system, and are water cooled.
A typical press cycle, once the press is loaded with one or more packs containing the laminate assemblies and press plates, entails closing the press to develop a specific pressure of about 1000–1500 psig, heating the packs at a predetermined rate to about 130–150° C., holding at that cure temperature for a predetermined time, then cooling the packs to or near room temperature, and finally relieving the pressure before unloading the packs on their carrier trays from the press. Those skilled in the art will have a detailed understanding of the overall pressing operations, and will recognize that careful control of the laminate's cure temperature and its degree of cure are critical in achieving the desired laminate properties (as are the proper selection of the resin formulations and papers used in the process).
After the pressing operation has been completed, and the press packs discharged from the press, the press plates are removed sequentially from the press pack build-up for reuse, and the resultant laminate doublets separated into individual laminate sheets. In a separate operation, these must then be trimmed to the desired size, and the back sides sanded so as to improve adhesion during subsequent bonding to a substrate. With a continuous laminating process, the trimming and sanding operations, and sheeting if desired, are usually done in-line directly after heat and pressure consolidation and curing between the rotating double belts. Conversely, with a conventional low pressure pressing operation, usually removal of unpressed surface paper edge “flash” is the only finishing step required.
As noted above, a relatively recent development in the building and design industries has been the growing widespread acceptance of using decorative laminates in flooring applications. Such flooring products, simulating stone or ceramic tiles, or wood planks, are most commonly produced either by adhering a conventional high pressure decorative laminate surfaced with a wear resistant overlay, as described in detail above, to a medium density fiberboard (MDF) or a premium grade high density fiberboard (HDF) substrate. Alternatively, the flooring composite material is pressed directly using a one-step low pressure process, again with an abrasive overlay protecting the decorative surface sheet and using MDF or HDF as the substrate. The fiberboard substrates are used in lieu of particleboard or other coarser, less expensive substrates due to the exacting machining requirements for the flooring product's tongue and groove or integral “snap lock” edge treatment joining systems that are most commonly used with these products. However, even with the more expensive HPDL clad flooring products, and using the best grades of “moisture resistant” HDF substrate (in which the board is produced at higher resin content with more moisture resistant resins), and even sized with wax and other “repellents”, serious application restrictions and problems persist with the current generation of these most widely used flooring products when exposed to repeated or prolonged contact with moisture or water. These deficiencies are due to their intrinsic hydrophilic, in fact hygroscopic, characteristics, as such products are comprised for the most part of cellulosic wood fibers. These deficiencies are compounded by the non-isomorphic, directional orientation of these fibers inherent to the papermaking and fiberboard manufacturing processes.
Indeed, even the best moisture resistant HDF grades will expand an average of about 0.075% along its machine direction (“MD”) and cross-machine direction (“CD”) for each 1% increase in its equilibrium moisture content. HDF in its original state, as produced by a mill and used by a flooring manufacturer, has an average moisture content of about 6%. With a non-moisture contributing subfloor, such as lauan plywood, under the best conditions of low relative humidity “RH” (˜10% RH) and high ambient temperature, the flooring HDF substrate moisture content will increase to about 7% (a +1% increase). On the other extreme, with the same type of subfloor and conditions of high humidity (˜90% RH) and low ambient temperature, the HDF substrate moisture content will increase to about 9% (a +3% increase). Typically, more moderate temperature and humidity conditions will result in an increase in the floor's HDF substrate moisture content to about 8% (a +2% increase). The practical consequences of this increase in the floor's HDF substrate moisture content, and resultant increase in its overall dimensions, are summarized in Table I below. The expansion figures shown below are an average of the expansion changes in both the MD and CD directions.
TABLE IExpansion WithMoistureRoom DimensionSubfloorRHTemp.ContentIncrease10 ft.20 ft.30 ft.HDF——6%————(from Mill)HDFLowHigh7%1%0.09″0.18″0.27″HDFMod.Mod.8%2%0.18″0.36″0.54″HDFHighLow9%3%0.27″0.54″0.81″
On the other hand, a traditional high pressure decorative laminate used as cladding (i.e., the laminated overlay, decorative and core layers) will lose moisture under low humidity conditions and shrink in both its MD and CD, and absorb moisture under high humidity conditions and grow in both its MD and CD dimensions. The NEMA specification LD 3-3.11 for dimensional change for VGS grade laminate (nominal thickness 0.028 inch “vertical grade standard”), which would typically be used to clad HDF for flooring applications, is 0.7% maximum in the machine direction and 1.2% maximum in the cross-machine direction in terms of total dimensional movement from low humidity conditions (less than 10% relative humidity at 70° C.) to high humidity conditions (90% relative humidity at 40° C.). Assuming equilibrium at ambient conditions of 50% relative humidity (midway for the test method), the laminate under high humidity conditions can grow 0.35% in the machine direction, and 0.60% in the cross-machine direction, with the consequences illustrated in Table II below:
TABLE IIExpansion WithRoom DimensionsRelative HumidityDirection% Change10 ft.20 ft.30 ft.10%MD−0.35−0.42″−0.84″−1.26″ %CD−0.60−0.72″−1.44″−2.16″50%MD0———50%CD0———90%MD+0.35+0.42″+0.84″+1.26″90%CD+0.60+0.72″+1.44″+2.16″
The relatively poor moisture resistance of the high pressure decorative laminate is primarily related to the phenol-formaldehyde (“phenolic”) resin impregnated core layer, in part because it comprises the majority of the laminate bulk and normally has a greater cellulose fiber to resin ratio than the surface components, and partly because of the more hydrophilic nature of “modern” water-solvated phenolic resin systems. Simply increasing the phenolic resin content in the core sufficiently to significantly improve moisture resistance is not practical since it would result in increased resin flow and bleed-out during pressing, as well as possible resin bleed-through into the laminate surface. Conversion to a more hydrophobic, organic solvent based modified phenolic resin is prohibited because of environmental considerations, and both alternatives are precluded because of their increased cost.
Thus, while the dimensional movement of the total floor assembly will be governed predominantly by the much greater mass of the HDF substrate, under high humidity and moisture, and in particularly wet, conditions, the greater movement of the flooring's HPDL cladding could warp convex and buckle the individual floor tiles or planks, lifting them off the subfloor.
Considering the recognized deficiencies in the current, most popularly used high and low pressure decorative laminate/HDF-based flooring products, they perform reasonably well in “small room”, low humidity, moisture and water environments (generally termed “residential applications”), where the effects of the compounded dimensional changes of the individual floor segments on the entire installation can be tolerated, if not controlled. Even with such installations, flooring manufacturers and installers typically recommend inclusion of (necessarily raised) expansion joints a minimum of every 20 feet to avoid buckling of the floor with any moisture uptake, although such expansion joints are aesthetically unattractive and physically intrusive. Accordingly, wet area installations, such as bathrooms, are not generally recommended.
Floor moisture protection is commonly attempted by recommending use of an underlayment between the subfloor and the new floor, which is typically comprised of foam materials sandwiched between polymeric films. These so called “floating floor” installations only help control the rate, not the total equilibrium amount, of moisture uptake from underneath the flooring panels and create the disadvantages of restricting spilled water drainage from above through the joints (thus permeating into the peripheral HDF substrate, which can cause severe swelling in those areas). Further, such installations impart a hollow sounding, springy feel to the entire floor when walked upon. The one important advantage of a floating floor installation, however, is that the foam inclusions act as shock absorbers and significantly improve the floor's impact resistance; the decorative laminate assembly itself having inherently very poor impact resistance if installed directly on a hard, rigid subfloor without the underlayment.
The deficiencies in existing decorative laminate are exacerbated when such conventional, decorative laminate clad HDF floors are installed on concrete (which is typical for commercial applications). The use of such existing decorative laminates in commercial applications has been largely avoided because of their aforementioned moisture and water sensitivity. Indeed, a newly poured and set concrete floor will typically generate about 14 pounds of water per 1000 square feet per day (14 lbs./1000 sq. ft./day), and HDF in contact with such a floor will reach an equilibrium moisture content of about 18%. Even an old, fully cured concrete floor on “dry” ground will continue to transmit water at an average rate of about 3 lbs./1000 sq. ft./day and result in a HDF moisture content of about 14%. Above about 12% moisture content in the HDF, the concern is not only dimensional change, but actual physical swelling and degradation of the fiberboard itself, as well as fungal and mildew damage. Furthermore, in areas with a high water table, such as southern Florida, where a typical house is built on a concrete slab without a basement, even old concrete transmits moisture at a rate similar to that for new concrete, with the same deleterious effects to HDF-based flooring. As such, these “wet area” residential and commercial flooring applications have largely been relegated to vinyl composition tiles and the like products. While they have the prerequisite moisture resistance and dimensional stability, by their very nature, they are quite soft and easily dented by heavy or impacted objects, and decorative designs are severely restricted to abstract stone-like patterns and the like.
U.S. Pat. No. 6,093,473 (“Min”) proposes a HPDL clad flooring assembly, utilizing a moisture resistant polymeric substrate (in particular, PVC), in conjunction with essentially a conventional high pressure decorative laminate cladding with the typical phenolic resin impregnated kraft paper based core, which only addresses part of the problem posed by conventional HPDL clad flooring assemblies (i,e., only addresses the problems associated with the HDF substrate).
A melamine-formaldehyde (“melamine”) surface resin, when sufficiently cured, has intrinsically good moisture resistance, as evidenced by the performance of such articles as molded melamine dinnerware. Thus, it is considered desirable to retain a melamine resin in the surface of an improved flooring product because of its moisture resistance as well as its other superior properties such as its color and clarity, hardness, heat and cigarette resistance, light stability and fade resistance, cleanability and optical compatibility with alumina inclusions required for enhanced abrasion and wear resistance. However, simply using a melamine resin, with its superior moisture resistance, in the core of the laminate, as well as in the surface, is precluded since they are most compatible with cellulosic, non-polymeric materials (which inherently degrade moisture resistance), and melamine resins are intrinsically brittle, such that the resultant laminate's stress crack and impact resistance would be deleteriously affected further, as would its machinability.
Further, while the use of an unsaturated and crosslinkable polyester “laminating” resin impregnated woven or non-woven glass, carbon or polymeric fiber cloth or mat, as is known in the art, could possibly improve moisture resistance and flexibility of the laminate core, this type of core would have several disadvantages. Such disadvantages would be relatively high cost, difficult processibility with conventional HPDL filler treating equipment, serious environmental problems, the core would still be comprised of a discontinuous moisture barrier, and such polyesters would be incompatible with the desired requisite melamine surface resin, curing by free radical rather than condensation polymerization. While the latter problem could be technically circumvented with use of a bridging agent or “tie sheet” as taught in U.S. Pat. No. 6,159,331 (“Chou”), which has both unsaturated polyester and melamine resin curing functionality, such materials are difficult to synthesize and expensive, and as such, best avoided if possible.
Accordingly, there remains a need for a moisture resistant and dimensionally stable decorative laminate assembly, and in particular, a decorative laminate cladding that can be used where there is repeated or prolonged exposure to moisture or water.
Further, thin, conventional decorative laminate claddings, with a phenolic resin impregnated kraft paper core, are by their very nature quite brittle and easily fractured. In the Min flooring assembly, where such a laminate is bonded to a PVC material (which is relatively soft and easily deformed), impact resistance is very poor. Indeed, a ball impact test of the product produced in accordance with Min results in instantaneous denting of the substrate and simultaneous circumferential cracking of the laminate cladding. Thus, there is a further need for a tougher, more impact resistant decorative laminate cladding.
Moreover, with traditional decorative laminate assemblies, it has been necessary to adhere the laminated cladding to the substrate through the use of an adhesive. Such adhesive, however, adds to the cost and complexity of manufacture of the decorative laminate assembly, typically requiring a separate processing step.
Accordingly, in view of the above, there is a need for a decorative laminate flooring assembly with improved moisture resistance and dimensional stability, as well as improved toughness, impact resistance and durability, that will offer a wide variety of design choice to the architect and consumer, and will reduce the cost and complexity of assembly. Such a decorative laminate assembly has not heretofore been provided.